This study aims at developing a non-invasive stable isotope tracer method to determine the kinetics of oxo-proline (OP), a substrate closely related to glutathione synthesis, and use OP kinetics as an indicator for the capacity of glutathione synthesis and the status of glutathione within the body of burn subjects and healthy controls. In healthy controls OP kinetics will be compared to cysteine kinetics after a 6-day period of generous sulfur amino acids intake and after a 6-day period of sulfur amino acid free diet.